Difference Between Radeon HD 4890 (RV790) And Radeon HD 4870 (RV770):
RV790 is nothing of an advancement in technology rather an advancement in RV770 graphics processor. RV770 and RV790 looks similar, but they are not identical, RV790 comes with a different layout of the die and the pin count is also not compatible with RV770 boards.
Beside higher clock speed ATI’s RV790 has improved shader efficiency, which will help RV790 to perform a bit faster than it’s predecessor RV770 even at the same clock and specification. ATI actually done something good with the shaders of RV790 to improve overall performance of the graphics processor. Higher clocks will require more power and 6+8 pin power connectors present in HD 4890 instead of 6+6 ones of HD 4870 is a sign of that.

Radeon HD 4890 Specifications:
So far the specifications that have leaked for this card are here:
HD 4890 has core clock speed of 850MHz, 256-bit wide memory interface, 1GB GDDR5 memory which is clocked at 975 MHz (effective 3900MHz Quad pumped), with memory bandwidth of 124.8GB/s up from the 111GB/s of its predecessor, the HD 4870. It has 800 stream processors, fabricated at 55nm, supports DirectX 10.1, SM4.1 and is CrossFireX ready.
These are reference clocks and we can expect even higher clocks with overclocked versions from ATI partners.

Catalyst Control Center
View of Catalyst Control Center with Radeon HD 4890 installed.

Catalyst Control Center

ATI reference design Radeon HD 4890:
While we are at this topic I should show you guys the upcoming Radeon HD 4890, it’s from ATI and it’s reference design, but we can expect some custom designed HD 4890 video cards after it’s official launch.